FIG. 1 illustrates a commercially available feed-forward radio-frequency amplifier. The amplifier is constructed in sections, indicated by dashed boxes, such as 3, 6, 9, and 12. When a section malfunctions, the symptoms manifest as faulty operation of the amplifier as a whole. To isolate the malfunction, a technician generally removes the individual sections from the amplifier, and tests each one.
Testing each section is not a simple task, because some of the sections are actually multi-port networks. For example, section 3 contains ports 15, 18, 21, and 24. For testing, the technician generally connects impedances to some ports of the section, but leaves two ports open. These impedances intend to (a) simulates the impedances seen by the section when installed in the amplifier and (b) create a pseudo-two-port device to test, using the unconnected two ports.
However, this simulation is an inexact procedure, and does not always replicate the actual impedances seen by the section. Thus, the section is frequently tested in an environment different from its operating environment. This difference can make diagnosis difficult.
In addition, because of the complexity of this testing procedure, it is generally done at a manufacturing site, rather than in the field at the site where the amplifier is located. Consequently, the testing requires shipping of components to the manufacturing site, which imposes added cost.